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 fingerstrips usually of beryllium copper, or magnetic strips under a brass
shimstock cover. Mesh gaskets take a set, are not self-cleaning or resilient,
and oxidize or otherwise corrode to degrade the electrical conductivity at
surface contact points. Therefore, mesh gaskets are not recommended.
Fingerstrips are preferred over gasketing because fingerstrips are typically
self-cleaning from the compression and wiping cycle in opening and closing the
door or hatch. Sufficient compression of the fingerstrips is required to
maintain a low resistance metal-to-metal contact from the door leaf to the
frame when the door is cycled closed. The knife edge type seal is also
available and uses beryllium copper fingerstrip in a door jamb recess. The
knife edge type door seal is less susceptible to damage than the
compression/wipe seal during normal operation but doesn't provide the
compression that the compression/wipe seal does. The magnetic type of seal is
less susceptible to damage during normal use than the seals using
fingerstrips, but the magnetic type has significantly lower shielding
effectiveness.
4.3.2
Vestibule/Threshold Protection. An electromagnetically shielded
vestibule is often required for the main personnel entrance to a TEMPEST
enclosure, with either interlocked or alarmed sets of doors so that the
shielded effectiveness is not violated when entering or exiting the
enclosure. Emergency exit or equipment doors, without vestibules, can be
provided and are normally designed to be opened only from the inside of the
enclosure. The thresholds on standard RF doors are raised and can limit
access. If the enclosure must be handicap accessible the threshold of the RF
door must be specially designed. All accessories, such as cypher locks,
doors, vestibules, door interlocking systems, combination locks, slide bars,
and emergency hardware should be provided by the shielding manufacturers.
4.3.3
Size and Location of Power Filters. When laying out the space
required for the enclosure, the architectural designer should carefully
consider the size and location of the electrical power and communication
filters. Power filters require adequate space, structural support and
ventilation for heat dissipation. Also, the filters should be mounted on the
shielded enclosure walls or as close to the enclosure as possible and should
be accessible after installation.
4.3.4
Shielding of Access Penetrations. The architectural designer must
also include raised access flooring and dropped ceiling into the enclosure, if
required, and ensure that there is adequate space above the ceiling and below
the flooring to install electrical and mechanical equipment. He is also
responsible for the design and specification of all coatings, finishes and
materials which may contact or penetrate the EMI shielding system so as not to
degrade the shielding system. It is recommended that the architectural
drawings include an EMI penetration schedule listing all electrical,
mechanical, civil, and structural penetrations of the shielding surface by
type and location with reference to the applicable disciplines drawings for
the actual penetration detail. This penetration schedule will be very
valuable to the ROICC for testing purposes during the construction.
4.4
Structural Design. Structural design considerations for EMI
shielded enclosures include welding details, support of the shielding system,
and electrical isolation of TEMPEST shielded enclosures from the building
framing members and foundation. The structural designer shall detail any
special design features or welding requirements necessary to prevent warpage
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